The present invention relates generally to computing systems with bus-connected peripheral devices, and more particularly to systems using small computer system interface (SCSI) bus structures.
An important and valuable part of any computing system today is its external storage facility. The presently preferred form of external storage is some type of hard disk structure because of its random access capability although other forms (e.g., tape, CD-ROM) are also used. This type of external storage is often used as (or called) mass storage. Mass storage which, as the name implies, is used to retain for later access huge amounts of data. When later accessed it is advantageous that the access be made as quickly as possible. While there are a number of bus architectures capable of connecting a processor unit to such mass storage devices, one that is very flexible and powerful, particularly for connecting hard disk mass storage devices, is the small systems computer interface (SCSI).
The physical SCSI bus structure comprises a set of conductive signal lines that carry data and control signals for information transfer between the devices on the bus. Signals and data are communicated according to pre-determined protocols. (See, for example, Working Document for Information Technology SCSI Parallel Interconnect 2 (SPI-2), ANSI X3.302.)
One well-known physical characteristic of SCSI bus structures is that the signal lines forming the bus are terminated by an SCSI bus terminator at each end of the bus. If there are multiple devices connected to the bus and the SCSI terminators are built into the last device at each end of the bus, then problems can occur when the last SCSI device on the bus must be removed or shut down. Removal/disconnection of the last device with the terminator degrades the signals communicated on the SCSI bus so that the entire system is brought down. In order to continue operation, the SCSI termination must be moved to the next in line last device on the bus.
This problem can also occur if the connection of the last device is inadvertently disconnected, the entire system will again be affected because the communicating ability of the SCSI bus is lost.